The crystalline lens of the human eye transmits and focuses light and is located behind the iris attached to the wall of the eye by suspensory ligaments known as the zonules. The lens consists of a more rigid central nucleus surrounded by peripheral cortical material which has a softer consistency. A fine membrane known as the capsule contains the entire lens. The eye has an anterior chamber and a posterior chamber.
Cataract formation refers to a loss of transparency of the crystalline lens of the eye and is a common occurrence with age. This results in a progressive reduction in vision, which can be restored with surgery. Modern cataract surgery involves removal of the cataractous lens and insertion of a plastics material intraocular lens to replace the crystalline lens. Further, modem cataract surgery uses ultrasonic energy to fragment and aspirate the cataractous lens by a technique known as phacoemulsification. When the cataractous lens is removed there remains in the eye a capsular bag remnant.
A central opening in an anterior portion of the capsule allows access to lenticular material by an ultrasonic probe, which typically has an outer wall and a central lumen. A plastics material sleeve surrounding a needle provides a conduit for transmission of fluid into the eye to replace fluid aspirated from the eye in removing the lens material. Once the harder nuclear material has been removed with the assistance of ultrasonic energy the softer cortical material can be aspirated with an irrigation/aspiration cannula.
In both phases of the procedure it is important that the anterior chamber of the eye is maintained at a positive pressure and constant volume to prevent collapse. Collapse of the anterior chamber can result in trauma to sensitive ocular tissues. Contact with endothelial cells lining a posterior surface of the cornea of the eye can result in irreparable damage. Even more common is inadvertent contact or aspiration of the posterior capsule, which can result in rupture of this fine membrane. The posterior capsule prevents escape of fluid known as vitreous humour, contained in the posterior chamber of the eye. Rupture of the posterior capsule and loss of the vitreous humour increases the risk of retinal detachment and cystoid macular oedema after cataract surgery with the subsequent loss of vision. Furthermore, if the posterior capsule is disrupted during surgery it may not be feasible to place an intraocular lens in the preferred position in the capsular bag remnant of the original natural lens. This too can have a less favourable outcome than is anticipated in uncomplicated surgery.
Thus, maintenance of a stable pressure and volume in the anterior chamber is very important when performing irrigation/aspiration operations, such as phacoemulsification, on the eye. In Australian Patent Number 684224 there is described and claimed a phacoemulsification needle whilst in Australian Patent 717553 there is described and claimed an intraocular irrigation/aspiration cannula. The inventions of these patents assist the surgeon in achieving this stable pressure and volume. The present invention seeks further to reduce fluctuations in chamber pressure and volume during irrigation/aspiration of the eye.
There are two basic types of pump systems that achieve aspiration of fluid and lens material during phacoemulsification and cortical aspiration. The first are positive fluid displacement pumps such as perstaltic pumps. In this system fluid flow is generated in tubing and significant vacuum is achieved when the tubing is occluded. In the other system typified by a venturi pump vacuum is generated in a cassette and the subsequent flow and aspiration of fluid from the eye is related to a preset vacuum.
In both systems the sequence of removal of nuclear and cortical material is similar. Fluid flow is generated in the aspiration tubing and fluid is aspirated from the anterior chamber via the phacoemulsification needle or irrigation/aspiration cannula. This attracts nuclear or cortical material to the needle or cannula and occlusion of the tip or aspiration port occurs. There is then a build up of vacuum in the tubing until the negative pressure generated by the pump overcomes the resistance of the lenticular material, which is then aspirated along the tubing. At this stage there is a rapid equalisation of pressure between the anterior chamber of the eye and the rest of the system with a rapid increase in fluid flow and drop in chamber pressure. This is typically referred to as post occlusion surge and often manifests as a forward movement of the posterior capsule as the anterior chamber pressure and volume alters.
The vacuum can be reduced by foot pedal control and the machines respond by venting or equalising the pressure in the system either to fluid or to air. The venting, however, occurs some distance from the Surgeon's handpiece and the anterior chamber and there is typically a time lag before the vacuum in the tubing is restored to atmospheric or a positive pressure and the pressure in the anterior chamber of the eye is restored to the normal resting or unoccluded level. Any measures which reduce the drop in anterior chamber pressure and shortens the time to attain the resting pressure, increases the safety of the surgical procedure and reduces the likelihood of inadvertent rupture of the posterior capsule or occurrence of other problems.